From German Patent Application No. DE 195 37 273 it is known to trigger wear-free, additional deceleration devices such as retarders with the aid of a speed control. Retarders, and engine brakes as well, may be used to maintain a constant vehicle speed on a downhill grade in those instances where an intervention in the engine control alone may be insufficient to maintain a slow speed. The additional deceleration devices must be differentiated from the service brakes of a vehicle.